perf(db): open MDBX DBIs only once at startup

[hai-rise]

Part 2 of #18292: Cache DBIs at startup time to reuse throughout the program's lifetime instead of just per transaction.

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reth-bench for the first 300 blocks of Shanghai:

$ reth-bench new-payload-fcu --advance 300

f66e197 (main):

2025-09-13T09:46:50.043037Z  INFO Total Ggas/s: 0.6499 total_duration=7.062721087s total_gas_used=4590139398 blocks_processed=300

This branch on f66e197:

2025-09-13T09:56:17.725214Z  INFO Total Ggas/s: 0.6967 total_duration=6.588003416s total_gas_used=4590139398 blocks_processed=300

Speedup:

• Ggas/s: 0.6499 -> 0.6967 ~ 7.2% higher execution throughput
• Duration: 7.062721087s -> 6.588003416s ~ 7.2% lower payload latency
• Best win: From 39.2ms down to 13.3ms payload latency

[hai-rise]

I tried &'static and it was ~0.3% faster than Arc, but reverted for consistency with the rest of the code base. We can review and make several things (like also the DB environment, etc.) &'static together later.

[shekhirin]

LGTM, nice! In theory we should even be able to make it a match, because tables are known during compile-time, but let's not bother with this for now.